Administration Seeking to Build Support in Congress on Iraq Issue
Bush administration seeks to shore up Congressional support for campaign against Iraq, even as it tries to head off pressure from Britain and France to work through United Nations before taking any action against Saddam Hussein; Vice Pres Dick Cheney says administration would 'consult widely with our Congress' before deciding whether to go to war; does not make clear whether administration would seek formal vote on authorization of war; seeks to undercut arguments of critics who urge return to United Nations weapons inspections, saying inspectors in past failed to uncover evidence that Iraq was in fact continuing to mass-produce deadly chemical agents; Security Council diplomats say United States has backed away from engaging in Council's pressure on Iraq about inspectors, with American diplomats calling such efforts increasingly fruitless; no other Security Council nations appear ready to give up on inspections process; photos (M)